Quenching and passivation of particulate metals

ABSTRACT

A process is disclosed for rendering active particles of reduced iron ores resistant to oxidation by treating the particles in a fluidized bed at elevated temperatures with mildly oxidizing gases.

United States Patent Esso Research and Engineering Company App]. No.Filed Patented Assignee QUENCHING AND PASSIVATION 01F PARTICULATE METALS10 Claims, 1 Drawing Fig.

11.8. C1 75/0.5 BA, 75/26 Int. Cl $2M 9/00, C21b 13/00 Field 01 Search75/0.5, 26

one ween Primary Examiner-L. Dewayne Rutledge Assistant Examiner-W. W.Stallard Attorney-Manahan and Wright ABSTRACT: A process is disclosedfor rendering active particles of reduced iron ores resistant tooxidation by treating the particles in a fluidized bed at elevatedtemperatures with mildly oxidizing gases.

PATENTEnu m 26 m1 ORE FEED REACTOR PASSIVATOR PASSIVATED PARTICULATEPRODUCT MM Inventors M14. 5

QUENCHIING AND PASSIVATIION or IPAIRTTCUILATE METALS BACKGROUND OF THEINVENTION This invention relates to the art of passivating particulatemetals, particularly ferrous metals, by forming protective films,surfaces or coatings on such metals.

There has been a longfelt need in the art for methods of cooling andpassivating particulate forms of metals. This is particularly true inthe iron and steel industry where, for example, direct ore reductionprocesses are becoming of increasing interest. In a direct reductionprocess, metallic iron is produced by subjecting iron ore, attemperatures below the melting point of the ore, to direct contact withhot reducing gas, or reducing gas mixtures.

Powdered metals withdrawn from direct iron ore reduction processes arehighly active in varying degrees and readily chemically react whenexposed to various environments. For example, powdered iron produced byreducing iron oxides with hydrogen-rich gases at relatively lowtemperatures tend to be highly pyrophoric and such product will burn ifdirectly exposed to air or other oxygen-containing gas, even atrelatively low temperatures. Iron ores that are reduced at relativelylow hydrogen concentrations and high temperatures tend to be lesspyrophoric on initial exposure but still possess and acute tendency,even after cooling, to be quite reactive. The metal can become severelyreoxidized, or back-oxidized, on continued exposure to anoxygen-containing gas, thus greatly decreasing its value. Moreover, whenthe reduced iron product is dampened or wetted, as by atmosphericmoisture, rain or spray, the problem can become even more seriousbecause hydrogen can be liberated. The hydrogen can, under certaincircumstances, ignite to produce fires.

Two reactions are believed primarily responsible for the oxidation, andignition, of a reduced iron product. A first reaction, which is onlyslightly exothermic, involves reaction between iron and water to produceiron oxides and hydrogen, as represented by the equations:

The second reaction, which is highly exothermic, involves reactionbetween iron and oxygen to ultimately form iron oxides as represented bythe following equations:

Fires may be caused when the reduced particulate iron product is storedunder circumstances such that the heat generated by the foregoingreactions cannot be sufficiently rapidly dissipated. Eventually, theheat generated from he hydrogen generation reaction builds up until thetemperature reaches a point where the air oxidation of iron becomes thecontrolling reaction. The latter reaction, being strongly exothermic mayignite the hydrogen, the combustion of which liberates still more heat,which sustains and increases the rate of the oxidation reaction. Underthese conditions, the reactions can continue until essentially all ofthe metallic iron has been rapidly back-oxidized to iron oxides.

Because of these and other difficulties, the particulate reduced ironproduct is generally compressed into dense compositions such asbriquettes, pellets or extrudates. This lessens the exposure area,reduces the amount of back-oxidation, and decreases the possibility offires though it by no means completely solves the problem unless veryhigh densities are achieved. Often, also, numerous various additives areincorporated with the particulate reduced iron product at or prior tothe time of compacting. The compacted product can also be surface coatedto lessen the exposure to moisture and oxygen.

The production of a passivated particulate reduced iron product suitablefor handling and shipping is a very desirable objective and wouldrepresent a great stride forward in the art. Attempts have been made topassivate the particulate reduced iron product from direct iron orereduction processes and thus avoid the necessity of compacting theparticles, but these attempts have not generally been successful. Wheresuch efforts have been made to passivate the particulate product, thepowdered metal is often heated in an inert, nonoxidizing, or reducinggas atmosphere. Thus, e.g., fresh reducing gases are contacted with thereduced iron, while careful efforts are made to avoid gas dilution orcontact with air which might make the circulating gas oxidizing towardmetallic iron. After sufficient cooling in such atmosphere, to lessenthe detrimental effects back-oxidation upon exposure to air, the productis withdrawn.

The foregoing and other disadvantages and difficulties associated withhandling and shipping such powdered metal products are known to the art.It has long been felt desirable to develop more effective ways and meansof quenching and passivatlng hot, active powdered metals, particularlyferrous metals such as those produced in direct iron ore reductionprocesses.

it is, accordingly, the primary objective of the present invention toprovide a method for passivating particulate metals. In particular, itis an object to obviate the foregoing and other disadvantages byproviding a method for passivating active metals and for quenching,cooling, and passivating such metals by forming protective films on themetals while in particulate form, especially ferrous metals. Moreparticularly, it is an object to render particulate metals passive andresistant to back-oxidation so that they can be handled and stored. Moreparticularly, it is an object to provide method for passivation offerrous metals produced by direct reduction processes. especiallyfluidized iron ore reduction processes.

SUMMARY OF THE lNVENTlON These and other objects are achieved inaccordance with the present invention which contemplates a new andimproved process for passivating, or quenching and passivating, activemetal particles of reduced iron ores by forming a bed of the particles,fluidizing the bed with a mildly oxidizing gas such as steam, or oxygendiluted with steam, or with inert gases such as nitrogen mixed withoxidizing gases such as steam or oxygen, or both, and maintaining thefluidized bed at temperatures ranging from about F. to about 650 F.,preferably about 150 F. to about 450 F., for a time sufficient to form aprotective oxide coating on the particles. When the metal particles areintroduced into the fluid bed at much higher temperatures, it isnecessary to quench the particles to maintain bed temperatures withinthe desired range. This is achieved by contacting the bed with spray ofatomized water. Preferably, the sizes of the droplets of water rangeless than about oneeighth inch in diameter, and more preferably lessthan about one thirty sixth inch in diameter. When such water sprayquenching is used, the temperature of the fluidized bed must bemaintained at from about 25 F. to about 200 F., and preferably fromabout 50 to about F., above the condensation temperature of the waterunder the conditions of operatron.

in its preferred form, reduced iron products, of a fluidizable particlesize distribution ranging principally about 10 to 10,000 microns,averaging about 50 to 400 microns, at temperatures ranging from about1000 F. to about 1800 F., and preferably from about l300 F. to about1600" F., are injected directly into a fluidized bed of the product. Theparticles can be injected mechanically, e.g., using hoppers, screwfeeders and the like, or pneumatically by entrainment in inert, mildlyoxidizing, or even reducing, gases. (When reducing gases are used, thevolume must be insufficient to render the overall fluid bed gascomposition nonoxidizing with respect to the metal surfaces of theparticles.) At the normally desired operating pressures which range ashigh as about 250 pounds per square inch, the bed is operated attemperatures ranging from about 225 F. to about 600 F., and preferablyfrom about 250 F., to about 450 F. Pursuant to these conditions,

the particulate metal is quenched or cooled by the water spray withoutcondensation on the surfaces.

Passivation of the particles is achieved by a thin film or surfacecoating of oxides believed to be substantially magnetite, i.e., Fe,0,,or magnetic oxide of iron which is formed by contact of the particleswith mildly oxidizing fluidizing gases. The thin film, which aids inprotecting the particles against further back-oxidation, results fromdeliberate partial reoxidation ranging up to about 3.0 percent, based onthe total weight of the particles. Generally, excellent passivation isachieved when such partial reoxidation is carried out to the extent ofabout 0.1 to 1.0 percent.

In general, fluidization is maintained by injecting a gas which ismildly oxidizing to the metal particle surfaces under conditionsexisting in the fluid bed. Preferably, fluidization is maintained withsteam or mixtures of steam and oxygen, or inert gases in mixtures withsteam or oxygen, or both. To avoid excessive reoxidation, the gasesshould contain no more than about 12 percent, and preferably about 1percent to 8 percent by volume, of free oxygen. 1f hot metal particlesare added to the fluid bed directly from a high temperature reductionprocess, the temperature rises and water is injected through an atomizerinto the bottom of the bed to maintain the desired bed temperature. Thevaporized droplets form steam which adds to the fluidizing gas.

The holding time of the reduced ore particles in the bed is sufficientlyshort to prevent excessive oxidation. Holding times range generally fromabout minutes to about 1 hour, and preferably from about 10 minutes toabout 40 minutes, depending upon the temperature and concentration ofoxidizing constituents in the fluidizing gases. In this manner, thereduced iron particles are quenched and made less reactive uponsubsequent exposure to conditions which would otherwise produceexcessive back-oxidation.

ln introducing the water into the fluidized bed for quenching of theparticulate metal it is essential that the water be in liquid state,preferably in the form of relatively fine droplets. The droplets aregenerally formed and introduced as a fine spray, preferably by atomizingin a stream of gas, most preferably fluidizing gas.

These and other features will be better understood by reference to theaccompanying schematic flow diagram to which reference is made in thefollowing detailed description BRIEF DESCRIPTION OF THE DRAWINGReferring to the drawing, there is illustrated a fluidized iron orereduction process, and process combination, of a preferred type forpassivating reduced ore particles.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring specifically to thedrawing, the combination contemplates a reactor 10 wherein oxidic ironores are reduced by countercurrent contact with ascending reducinggases. A gas regeneration and recovery facility 20 is also shown, thisproviding for the recovery of spent reducing gas. A passivator 30provides for the passivation, or cooling and passivation, of theparticulate reduced iron product withdrawn from reactor 10.

In the iron ore reduction portion of the process, oxidic iron ore, e.g.,hematite, is charged into the top stage, i.e., stage 1, of reactor 10via line 11. The iron oxides descend through, and are progressivelyreduced in the sequence of stages 1, 2, 3, 4, and 5, each of whichcontains one or more beds of fluidized ore at progressively decreasingdegrees of oxidation, The individual beds or stages are operated at thesame or at different elevated temperatures ranging generally from about1000 F.

to about l800 F., or more generally from about 1300 F. to about 1600 Fparticularly in the ferrous reduction stages. In the first stage, orstages, the oxides are generally preheated. Fuel can be burned withoxygen, e.g., in the top bed or second bed to accomplish this, underoxidizing or reducing conditions. The oxides are thence reduced from theferric oxide state to magnetic oxide of iron and in a subsequent stage,or stages, from magnetic oxide of iron to ferrous oxide. Finally,

in the last stage, or stages, the ore is reduced from ferrous oxide tosubstantially metallic iron. 1n the final reduction stage the-reducediron product is generally from about 50 to about 95 percent metallized.

Reducing gas is introduced into reactor 10 countercurrent to the flow ofore. Hot ascending reducing gas, consisting generally of carbon monoxideor hydrogen, or both, is thus introduced into the bottom of reactor 10via line 12, the gas sequentially contacting fluidized beds in stages 5through 1. Cyclone separators (not shown) are generally provided betweenthe beds to remove entrained particles from the ascending fluidizinggas.

Spent gas is withdrawn from reactor 10 via line 13. The gas can bevented, but preferably most of the gas is regenerated by removal of theoxidized components, generally water and carbon dioxide. The gas is thusregenerated by passage through line 14 to the recovery facility 20 wherewater is precipitated and removed by cooling and the remaining gas isscrubbed, e.g., with basic compounds such as monoethanolamine, to removecarbon dioxide. The regenerated gas is then passed via lines 21 and 22,combined with fresh reducing gas from line 23, reheated via means notshown, and thence injected via line 12 into the bottom of reactor 10.

Particulate iron metal solids are withdrawn from the last stage ofreactor 10, i.e., stage 5, via line 31. The particulate metal, ofparticle sizes averaging generally from about 50 to about 400 microns,and as large as about 14 mesh (Tyler Series), is discharged into line32, wherein it is entrained by a fluidizing gas stream and thence fedinto the bottom of passivator 30.

The particulate metal solids are introduced at high temperatures intothe lower part of fluidized bed 33 in passivator 30. The bed is in agenerally turbulent fluidized state, and the solids are there contactedby an atomized spray of water introduced into the bottom of passivator30 via lines 34, 35, and 36. Upon introduction into the fluid bed 33,the solids are instantly quenched and the water droplets are vaporizedto form steam. The lower portion of the passivator 30 is characterizedas preferably having a section having a lengthzdiameter ratio ranginggenerally from about 2:1 to about 6:1, and more preferably from about3:1 to about 4:1, to provide a relatively high velocity gas, ranginggenerally from about 1 foot per second to about 5 feet per second, andpreferably from about 2 feet per second to about 3 feet per second, forcreation of turbulence. The velocity of the gases through the upperportion of bed 33 ranges from about one-tenth to about one-half thisvelocity, and preferably from about one-fourth to about one-fifth thisvelocity.

The solids, after entry into the lower portion of quench passivator 30,are passed into the upper portion of fluidized bed 33. The passivatedparticulate product, after sufficient holdup time, is removed from theupper portion of fluidized bed 33 via overflow line 37 as a passivateddry particulate product.

Effluent gases are removed from the quench passivator 30 via line 38 andintroduced into the cyclone separator 39. Gases from the cycloneseparator 39 are exhausted via line 40, and the entrained solids arerecovered by return through line 41 to line 37 from whence they arerecovered as a portion of the passivated particulate metallic ironproduct. Alternatively, the solids from the cyclone can be recycled tothe passivator if additional passivation is required.

The following examples are illustrative of the effectiveness ofpreparing a passivated particulate product from the abovedescribedprocess.

EXAMPLE 1 Particulate reduced iron ore 95.0 percent metallized iswithdrawn from the final stage of the reactor 10 at 1400 F. andentrained in a gaseous mixture of nitrogen containing 4 volume percentoxygen and injected into the bottom of the passivator 30, maintained atessentially atmospheric pressure. The metal is there contacted by anatomized spray comprising 67 percent water and 33 percent nitrogen. Theiron particles are quenched to fluid bed temperatures of about 420 F.while the water droplets are instantly vaporized to steam, whichcomprises about 30 percent of the total fluidizing gases. After a holduptime of 16 minutes at 420 F., the dry particulate product is dischargedfrom the fluidized bed of the passivator. It is then cooled toatmospheric temperature in nitrogen. The oxide coating on the product issufficiently thin that metallization is reduced by only 0.3 percent to94.7 percent.

To determine the degree of passivity of the so-treated particles,analyses are performed to measure the amount of hydrogen generated, uponimmersion in water, and metallization loss upon exposure to theatmosphere for three days. The hydrogen generation measurements arecalculated on the basis of standard cubic feet of hydrogen generated perhour per ton of reduced iron product, upon immersion in water, at l25 F.Analyses are also performed on an untreated or unquenched portion of theproduct taken directly from line 31 and cooled gradually in reducing gasto ambient temperature. Comparisons of the results are made. Inaccordance therewith, it is found that considerably less hydrogen isliberated by the quenched passivated metal, which also loses much lessmetallization. This is shown by reference to the data given in thefollowing table, wherein treatment A refers to the unpassivated productand treatment 8" refers to the passivated product.

Particulate iron of 88.0 percent metallization is withdrawn from thefinal stage of the reactor 10 and entrained in essentially pure nitrogenand injected into the bottom of passivator 30 at essentially atmosphericpressure. Therein it is treated precisely as in example 1, except thatit is held for 46 minutes in the fluid bed at 409 F. Comparison ofunpassivated and passivated samples, Aand l3, again show the excellentpassivation achieved by the method of the present invention.

Metallizntion Hydrogen Treatment Loss Generation A 3-4'1 l 1.0 SCFH/TonB 0.4% 0.4

EXAMPLES 3 and 4 These examples illustrate the effectiveness of thepresent invention even when the water spray quench is not used.

Samples of reduced iron ore previously prepared and cooled in an inertatmosphere are introduced through a conventional lock hopper intopassivator 30. The articles are heated and fluidized by a preheatedfluidizing gas consisting of nitrogen and oxygen under conditions andfor times indicated below. As the following table indicates, excellentpassivation is achieved:

Example No. 3 4 Fluidizing Gus N,+4.5'1 O, N,+8k 0 Fluid Bed Temp. 357IF. 2 l 7 IF. Holding Time. Minutes 44 50 Product Metallization, 1'

Before Passivution 94.8 96.0

After Pumivat'ron 94.4 96.0

Metnllization Loss 0.4 (less than (H Hydrogen Generation, SCFH/T onBefore Pssivution ll 7.6 7.9

After Pussivation 0.0 0.0

in sharp contrast to the foregoing, when samples are treated in thepassivator inprecisely the same manner, but without any oxygen in thenitrogen, the resulting product is extremely active, generating hydrogenat essentially the same rate as the untreated samples.

EXAMPLE .5

Reduced iron ore is prepared, and a portion'of it is passivatedessentially as in example 4.The unpassivated portion is dumped into aconical-shaped pile, exposed to the atmosphere, initially at atemperature of l20 F. The passivated portion is dumped at 2l0 F. into anadjacent pile virtually identical in size and shape. Both piles arepenetrated at several points with thermocouple probes. Oxidationgenerates heat and causes increased temperatures in the unpassivatedpile, while no increase is seen in the passivated pile, as shown below:

Reduced iron ore is prepared, and a portion of it is passivatedessentially as in example 1. The unpassivated and passivated portionsare subdivided into smaller portions of equal size, which are preheatedwith drynitrogen to various initial temperatures and dumped intoidentical lPlLES. The piles are equipped with thermocouples and thetemperatures are continuously recorded to observe the effect of anyback-oxidation. The results are given below:

Sample Initial Temp. Maximum Temp.

Unpassivated F. l70 F. F. 240 F. 220 F. 380 F. Pussivuted l75 F. F. 230F. 230' F. 240 F. 240 F The advantages gained by the present process areapparent.

The advantages gained by the present process are apparent.

it is apparent that the invention is subject to various changes andmodifications without departing its spirit and scope.

Having described the invention, what is claimed is:

l. A process for passivating active metal particles of reduced iron orecomprising:

forming a bed of the particles;

7 fluidizing the bed with a mildly oxidizing gas selected from the groupconsisting of steam, steam diluted with inert gas, oxygen diluted withinert gas, oxygen diluted with steam. and mixtures of these gases; and

maintaining the bed at temperatures ranging from about 90 F. to about650" F. for a time sufficient to form a protective oxide coating on theparticles.

2. The process of claim 1, wherein said bed temperatures range fromabout 150 F. to about 450 F.

3. The process of claim I wherein said metal particles are continuouslyadded to said bed at temperatures above about 650 F. and are quenched tobed temperatures ranging from about 225 F. to about 600 F., by sprayingliquid water into said bed.

4. The process of claim 3, wherein said particles are quenched totemperatures ranging from about 250 F. to about 450 F.

5. The process of claim 3, wherein said particles of reduced ore areintroduced into said bed at temperatures ranging from about 1000 F. toabout [800 F. and comprise principally particles ranging from about l0to about l0,000 microns. averaging about 50 to about 400 microns, insize.

6. The process of claim 1. wherein up to about3.0 percent of the metalin said particles is reoxidized to form an oxide film which protects theparticles against further oxidation. I

7. The process of claim 6 wherein from about 0.l percent to about 1.0percent of said metal is reoxidized.

8. The process of claim 2 wherein said mildly oxidizing gas contains upto about 12 volume percent t'ree oxygen.

9. The process of claim 8 wherein said mildly oxidizing gas containsabout 1 to about 8 percent free oxygen.

10. The process of claim 1 wherein said particles are maintained in saidbed for holding times ranging from about 5 minutes to about I hour.

2. The process of claim 1, wherein said bed temperatures range fromabout 150* F. to about 450* F.
 3. The process of claim 1 wherein saidmetal particles are continuously added to said bed at temperatures aboveabout 650* F. and are quenched to bed temperatures ranging from about225* F. to about 600* F., by spraying liquid water into said bed.
 4. Theprocess of claim 3, wherein said particles are quenched to temperaturesranging from about 250* F. to about 450* F.
 5. The process of claim 3,wherein said particles of reduced ore are introduced into said bed attemperatures ranging from about 1000* F. to about 1800* F. and compriseprincipally particles ranging from about 10 to about 10,000 microns,averaging about 50 to about 400 microns, in size.
 6. The process ofclaim 1, wherein up to about3.0 percent of the metal in said particlesis reoxidized to form an oxide film which protects the particles againstfurther oxidation.
 7. The process of claim 6 wherein from about 0.1percent to about 1.0 percent of said metal is reoxidized.
 8. The processof claim 2 wherein said mildly oxidizing gas contains up to about 12volume percent free oxygen.
 9. the process of claim 8 wherein saidmildly oxidizing gas contains about 1 to about 8 percent free oxygen.10. The process of claim 1 wherein said particles are maintained in saidbed for holding times ranging from about 5 minutes to about 1 hour.